The oil and gas industries utilise a technology known as hydraulic fracturing or “fracking”. This normally involves the pressurisation with water of a system of boreholes in oil and/or gas bearing rocks in order to fracture the rocks to release the oil and/or gas.
In order to achieve this pressurisation, valves may be used to block off or isolate different sections of a borehole system. These valves are referred to as downhole valves, the word downhole being used in the context of the disclosure to refer to an article that is used in a well or borehole.
Downhole plugs are one type of valve. A conventional plug consists of a number of segments that are forced apart by a conical part. The cone forces the segments out until they engage with the pipe bore. The plug is then sealed by a small ball. Another way of forming such valves involves the use of spheres (commonly known as fracking balls) of multiple diameters that engage on pre-positioned seats in the pipe lining. Downhole plugs and fracking balls may be made from aluminium, magnesium, polymers or composites.
A problem with both types of valve relates to the strength of the material used to make them. An essential characteristic of the material is that it dissolves or corrodes under the conditions in the well or borehole. Such corrodible articles need to corrode at a rate which allows them to remain useable for the time period during which they are required to perform their function, but that allows them to corrode or dissolve afterwards.
The applicant's earlier patent application, GB2529062A, relates to a magnesium alloy suitable for use as a corrodible downhole article. This document discloses alloys containing 3.3-4.3 wt % Y, up to 1 wt % Zr, 2.0-2.5 wt % Nd and 0.2-7 wt % Ni which have corrosion rates of around 1100 mg/cm2/day in 15% KCl at 93° C. (200 F). The alloys have a reasonable yield strength (around 200 MPa) and an elongation (ie ductility) of around 15%. However, the range of uses of these alloys are limited by their strength.
One known approach for strengthening magnesium alloys containing Y (and optionally a rare earth metal other than Y) is to use precipitation hardening or ageing to increase the yield strength of the alloy. For example, a T5 ageing process may be used. However, this approach is not effective for the super corroding alloys described in GB2529062A. This is thought to be due to the interference between the age hardening response and the alloy additions required to enhance the corrosion properties.
A material which provides the corrosion characteristics required for downhole valves, but with improved strength, has been sought.